Allow some interaction between LLVM and FIR dialect by allowing
conversion between FIR memory types and llvm.ptr type.
This is meant to help experimentation where FIR and LLVM dialect
coexists, and is useful to deal with cases where LLVM type makes it
early into the MLIR produced by flang, like when inserting LLVM stack
intrinsic here:
0a00d32c5f/flang/lib/Optimizer/Transforms/StackReclaim.cpp (L57)